The present invention relates to amplifier circuits and in particular to amplifier circuits that convert a single-ended input signal into a differential output signal.
Signal processing circuitry, especially for radio receivers and the like, usually requires a differential input signal. It is common in many electronics applications to require the conversion of a single-ended signal into differential form so that it can be properly handled by whatever signal processing circuitry is used. This conversion is typically carried out using circuits such as that shown in FIG. 1.
FIG. 1 shows a single-ended input, differential output amplifier formed from an emitter coupled transistor pair. Current source 115 draws current from the emitter electrodes at npn transistors 107 or 108 by way of resistors 110 and 111 respectively.
As is well known, an increase in the signal input to the base electrode of transistor 107 will effect an increase in the current flowing from the collector electrode of transistor 107 and a decrease in the current flowing from the collector electrode of transistor 108. The opposite effect occurs when there is a decrease in the input signal. The collector currents of transistors 107, 108 form a differential output signal at output terminals 101, 102. The output signal will be balanced if current source 115 is implemented as a constant current source.
Although the amplifier of FIG. 1 offers reasonable linearity and a balanced output, the circuit has poor noise properties. These noise properties are due largely to the thermal noise of resistors 110, 111, which create noise directly in the main current paths. Current source 115 will also introduce noise into the output signal because it experiences quite large voltage swings at its connection with resistors 110 and 111. A significant amount of noise will appear at output terminals 101, 102 as a result of transistors 107, 108 having their base resistances in series.
Because the single-ended to differential signal converting amplifier stage is usually used to prepare raw input signals for signal processing, reductions in noise figure made in this amplifier circuit have a greater effect than noise figure reductions made elsewhere. There exists a need for a single-ended input, differential output amplifier circuit with improved noise properties.